The present invention relates generally to electrical test probes for oscilloscopes and other measuring, monitoring and signal processing instruments and more particularly to an improved electrical test probe having integral strain relief and ground connection.
A commonly used design for electrical test probes is an electrically conductive elongate tubular body made of nickel plated brass having a substrate disposed therein. The substrate contains either passive or active circuity to minimize probe loading and to terminate the probe in the characteristic impedance of the measuring device. The elongate tubular body has a probing tip at one end that is secured in the tubular body by an insulating plug. The probing tip extends through the insulating plug and is exposed within the tubular body. The substrate is electrically connected to the probing tip by electrically conductive elastomeric material or other such conventional connecting methods. Surrounding a substantial portion of the tubular body is an insulating material, such as injected molded plastic parts. Various methods are used for attaching the plastic parts to the tubular body, such as press fitting, gluing, or injection molding of the plastic directly onto the body. The plastic parts are generally formed with an outwardly extending flange that serves as a finger stop and guard. The plastic parts also serve as an outer housing for securing a transmission line, or cable, to the probe.
The transmission cable has a central conductor encased in a dielectric material and surrounded by an outer conductive shielding material. The outer conductive shielding material is covered by an insulating material. The central conductor is electrically connected to the substrate and the outer shielding material is electrically connected to the tubular body. Many methods are employed for connecting the transmission cable to the electrical test probe.
One such method is described in U.S. Pat. No. 4,209,742 entitled "Modular Probe System" and assigned to the assignee of the present invention. The transmission line in the modular probe system is secured in a connector plug that mates with the tubular body of the probe. An injected molded plastic housing is formed around the tubular body having a central bore into which the tubular body extends. The connector plug fits into the central bore and a pair of coaxially disposed contact members separated from each other by a dielectric material mate with the probe tip and the tubular body. The inner contact member is electrically connected to the central conductor of the transmission line and the outer contact member is electrically connected to the outer conductive shield. The outer contact member is supported in the plug by an integral annular member. A metal sleeve is crimped over the inner end of the annular member and an end portion of the cable to anchor the contacts to the cable. An insulating cove is formed over the plug.
A major drawback to this design and other types of designs where bushings, threaded connectors, and the like are used is the number of components needed for assembling the probe and the time required to manufacture the probe.
What is needed is a simple, low cost electrical probe that requires a minimum number of parts for connecting the transmission cable to the probe while at the same time providing the necessary electrical connections and strain relief for reliability.